The present invention relates to methods of preparing porous plaques for use as electrodes in a variety of applications. One principal use is for the preparation of porous cathodes to be employed in a molten carbonate fuel cell. A plaque of the type described herein may also have application as a target within an apparatus for production and recovery of tritium produced in the neutron bombardment of lithium compounds. Various other applications may occur in which a gas is reacted or generated within a porous substrate.
Molten carbonate fuel cells typically operate at high temperatures of about 900-1000 K. to convert chemical energy to D.C. electricity. Fuels such as H.sub.2, CO or methanol react with oxidant gases, for instance, air or oxygen including carbon dioxide during this production of electrical energy. Typical reactions are as follows:
______________________________________ at the anode H.sub.2 + CO.sub.3.sup.-- yields CO.sub.2 + H.sub.2 O + 2e.sup.- CO + CO.sub.3.sup.-- yields 2CO.sub.2 + 2e.sup.- at the cathode 2e.sup.- + CO.sub.2 + 1/2 O.sub.2 yields CO.sub.3.sup.-- ______________________________________
It is contemplated that these fuel cells will typically operate in stacks of repeating elements. Each element contains an anode, a cathode and an electrolyte structure separating the two electrodes. In the molten carbonate cell, anode structures typically include porous sintered nickel, for instance, alloyed with chromium or cobalt for strength. The electrolyte structure, for instance, can be a porous tile of lithium aluminate filled with a molten carbonate electrolyte. The electrolyte tile should include an appropriate pore structure to permit wetting without flooding of the adjacent electrode with the electrolyte. Typical electrolyte tiles and molten carbonate salt electrolytes are presented in U.S. Pat. No. 4,115,632 to Kinoshita et al and in U.S. Pat. No. 4,251,600 to Sim et al.
Early efforts in providing cathodes for molten carbonate fuel cells have involved assembling the cell with a porous nickel plaque as the cathode. On raising the cell to operating temperature and exposing the plaque to oxygen and molten carbonate salt, the nickel cathode plate oxidizes in situ to NiO and incorporates a small percentage e.g. 2-3% of lithium. Nickel oxide (NiO) is a deficient semiconductor (P-type) that exhibits rather poor electrical conductivity. The incorporation of lithium ions within the structure has been found to provide an enormous enhancement in the P-type conductivity. However, substantial difficulties have arisen in the preparation of nickel oxide cathodes with in situ oxidation and lithiation of the sintered nickel plaque. For example, substantial cathode swelling with accompanying lithium loss from the electrolyte has made it desirable to provide alternate approaches to the preparation of porous cathodes for the molten carbonate fuel cells.